This application claims the priority of JP8-26,1747, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for cleaning exhaust gas released from an internal combustion engine, particularly to a method for removing nitrogen oxides efficiently from the exhaust gas. The present invention also relates to a catalyst for removing the nitrogen oxides.
The exhaust gas released from the internal combustion engine. of a vehicle contains generally carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NO.sub.x), and other compounds, which are hazardous to the human body, and cause pollution. A great deal of effort has been devoted to decreasing the amount of these substances released from an engine. In addition to efforts to improve combustion conditions and decrease the amount of these substances released from an engine, methods for cleaning released exhaust gas, the method using both a three way catalyst, primarily composed of Pt, Rh, Pd, and the like, for oxidizing HC and CO, and, at the same time, reducing NO.sub.x to make them harmless is the most common. The three way catalyst is aimed at cleaning the exhaust gas, of which the air to fuel ratio (obtained by A(weight of air)/F(weight of fuel)) is set approximately at a stoichiometric ratio (14.7).
However, in order to decrease fuel consumption by the engine, a lean-burn engine, which is operated with an A/F leaner than the stoichiometric A/F, has currently been developed. As one, of the conditions of the lean-burn engine, the content of oxygen in the exhaust gas from the lean-burn engine is increased. Therefore, NO.sub.x cannot be removed efficiently, because the catalytic activity of the three way catalyst in the presence of oxygen is low. Accordingly, as a NO.sub.x removing technology for supporting the lean burn engine, a catalyst which reduces NO.sub.x with unburned hydrocarbons or carbon monoxide (called the lean NO.sub.x catalyst hereinafter) to eliminate the NO.sub.x from the exhaust gas in the presence of oxygen has been developed. For instance, a catalyst, which is composed of transition metals such as copper and the like are supported on a carrier such as zeolite, is disclosed in JP-A-1-130735 (1989) and JP-A-1-266854 (1989). An utilization system of the catalyst of the same is disclosed in JP-A-3-74514 (1991).
One of the problems to be solved in the development of the lean NO.sub.x catalyst is adding SO.sub.x resistivity to the catalyst. Fuel, such as gasoline, and the like for the internal combustion engine, contains sulfur to the amount of from ten ppm to as much as 1000 ppm. Sulfur generates SO.sub.x, which causes catalyst poisoning. SO.sub.x is composed of SO.sub.3 and SO.sub.3. In the case of removing NO.sub.x from the lean-burn exhaust gas, SO.sub.2 is readily oxidized to SO.sub.3 because of the presence of oxygen. Therefore, the catalyst is readily poisoned by SO.sub.x and the addition of the SO.sub.x resistivity is indispensable for the lean NO.sub.x catalyst. Hitherto, a method for fixing SO.sub.x by forming a complex sulfate which is readily decomposable (JP-A-7-51544 (1995)), and a method for suppressing the oxidation of SO.sub.2 (JP-A-7-171349 (1995) ) have been disclosed.
Furthermore, because the catalyst for removing hazardous substances from exhaust gas is used in an atmosphere at a high temperature in the range of approximately 300.degree. C. to 500.degree. C., the catalyst is recquired to have a sufficient catalytic activity even after being exposed to the above temperature condition for a long time, and concurrently, to have heat resistivity resisting temperatures in the range of 800.degree. C. to 900.degree. C., temperature conditions of high speed driving.